The present technology relates to core/shell nanowires, specifically, a cadmium-selenide core and a zinc-sulfide shell. The present technology also relates to batch methods and continuous flow methods of making core/shell nanowires.
Nanoscale materials, such as nanoparticles and nanowires, are the subject of considerable research in materials science. Part of the interest stems from the fact that materials formed with a nanometer scale dimension frequently exhibit properties that are dramatically different from the corresponding bulk materials, including unique optical, electronic, and mechanical properties. Nanowires may be used in a wide variety of applications such as various integrated circuits, chemical and biological sensors, optoelectronic devices, and photovoltaics. For example, there is an interest in developing semiconductor nanowire-based devices for use in various electronic devices and systems to replace or supplement complementary metal-oxide-semiconductor (CMOS) technologies. As another example, nanowires have a large surface area, and thus may be advantageously applied to various sensing modalities and configured as particular sensors, e.g., for biological analytes and other analytes. High surface area also is beneficial for catalysis and photocatalysis.
Of particular interest are core/shell nanowires, such as CdSe nanowires (core) coated with a wide bandgap semiconductor (shell). Both the core and the shell compositions for nanomaterials (nanocrystals, nanorods, and nanowires) are generally type II-VI, IV-VI, and III-V compound semiconductors (CdS/ZnS, CdSe/ZnS, CdSe/CdS, CdS/CdSe, and InAs/CdSe for example) [1-4] and elemental semiconductors such as group IV (Si/Ge, and Ge/Si for example). [5,6] Core/shell semiconductors nanomaterials can be classified as either Type I or Type II, based on the relative positions of the conduction and valence bands of the core and shell materials. In Type I, the conduction and valence band edges of the core lie within the bandgap of the shell. In Type II, both the conduction and valence band edges of the core are either higher or lower than the band edges of the shell. These shells have been shown to eliminate surface defects and improve photo- and air-stability of quantum dots and nanorods. [7-9]
Core/shell quantum dots and nanorods are studied for a wide variety of applications, including photovoltaics, [10-13] chemical sensors, [14-16] lasing, [17] and photocatalytic water splitting. [18] CdSe/ZnS core shell quantum dots have been widely explored, [2,19-21] and there have also been a few reports on CdSe/ZnSnanorods. [8,17] These nanorods show enhanced lasing compared to quantum dots, and have potential for other photonic applications such as LEDs. [17]
It is interesting to note that the development of core/shell quantum dots was critical for the successful commercialization of quantum dot based fluorescent tags. [7] The stabilizing influence of a ZnS shell to a CdSe quantum dot leads to a more emissive and more water stable nanoparticle. These features have made CdSe/ZnS quantum dots popular for biological applications such as cellular imaging. [7] In addition to the improved photo- and air-stability, CdSe/ZnS quantum dots are more robust in aqueous environments and have an enhanced quantum yield of fluorescence. [2,7,21]
Materials with type I heterojunctions have been shown to have substantial photocatalytic activity, [22] which is most likely to arise from passivation of defect states (which trap electrons) of the core material by the wide band gap shell (which also is what leads to enhanced emission). This, in addition to rapid tunneling of electrons from the core through the thin shell, allows the electrons to access the solvent, and perform photocatalysis. [22] CdSe/CdS materials have attracted wide-spread interest for their photocatalytic properties, and particularly for their ability to generate hydrogen from water. [22]
While numerous methods for coating quantum dots (QD) with other semiconductors can be found in the literature, there are few examples of coating nanowires (NWs) [1,9,23-27] or nanorods (NRs). [8] The few reports of core/shell nanowires have described tedious optimizations of many reaction parameters. Extremely good results have been obtained for QD core/shell production via microreactor. [2,3,20] However, application of these procedures and processes to coat nanowires has not been successful.
Semiconductor nanowires may be synthesized using a range of conditions, from high temperature (e.g., less than 1100° C.) gas-phase reactions, [28] to relatively low temperature (e.g., less than 250° C.) solution-phase conditions. Solution-phase routes to semiconductor nanowires are of particular interest due to the potential for size and shape control, chemical surface passivation, colloidal dispersibility, and adaptability to high throughput continuous processes. Solution methods allow greater control over structure and function than gas phase methods. [29]
Solution-phase routes to semiconductor nanowires include non-catalyzed (e.g., oriented attachment, and solvothermal/hydrothermal growth), and catalyzed (e.g., supercritical-fluid-liquid-solid growth, and solution-liquid-solid growth) approaches. The most reliable, reproducible, and general method has proved to be the catalyzed method, and consequently this method has been widely adopted. [30] In the catalyzed approach, small metal droplets are used to induce the asymmetric crystallization of semiconductors from precursors in solution. [31] The metal droplets are interchangeably called the metal seeds, the metal seed particles, or the catalyst. Among the catalyzed solution-phase routes, solution-liquid-solid approach affords the use of lower temperatures (e.g., less than 250° C.) to grow semiconductor nanowires with such advantages as nanowire crystallinity, length, and diameter control. [30] The solution-liquid-solid (SLS) is so named based on a proposed mechanism wherein the nanomaterials precursors are in solution, then partition in to the liquid catalyst, and lastly the solid nanowire is grown.
Common among the solution-phase routes to many nanomaterials is a hot-injection technique in which a precursor solution at room temperature is injected into a second precursor solution held at a certain elevated temperature in order to rapidly produce a large amount of monomers to trigger a burst of nucleation and subsequent growth of nanomaterials in a controlled manner in the reaction system. [32] The solution-phase routes are typically performed as a small scale batch process to achieve the required control over thermal and mass transport properties needed to control nucleation and growth.
Despite the fact that quantum dots and nanowires can both be made by the hot-injection batch method, and many examples of core/shell quantum dot solution syntheses have been published, there has been little success in adapting those procedures to produce core/shell nanowires. Solution-phase methods for producing core/shell nanowires are difficult to implement, as many of the chemicals used in the core synthesis impede addition of the shell semiconductor. The Kuno group found that the presence of trioctylphosphine oxide (TOPO) or other coordinating solvents strongly impeded the coating process, [1] and thus necessitated washing and ligand exchange to remove excess TOPO from the nanowires. However, washing can lead to bundling, the phenomenon where nanowires stick together, and there is evidence that the bundling of nanoparticles induced by washing impedes the coating process. [33] In contrast, no washing step is required for core/shell quantum dot [2,3] or quantum rod [8] synthesis.
Additionally, the reaction conditions used to add the shell material frequently can damage the core nanowires. For example, nanowires and nanorods are often found to be etched to smaller dimensions under some reaction conditions used to coat quantum dots. [8,9] Lastly, the chemicals used to create materials of high interest, such as CdSe/CdS core/shell nanowires, are frequently extremely dangerous, such as the highly reactive dimethyl cadmium. Dimethyl cadmium has been successfully used to produce CdSe/CdS core shell nanowires [1], but its strongly pyrophoric nature necessitates the use of a glovebox and air-free conditions.
Other core/shell nanomaterial compositions of high interest include CdSe/ZnS. However, CdSe nanowires have yet to be coated with a zinc sulfide shell. Previous attempts by Li et al. at producing CdSe/ZnS nanowires using zinc hexadecyl xanthate were unsuccessful, and resulted in ZnS nanorods not attached to the CdSe parent nanowires. [9] The difficulty in synthesizing CdSe/ZnS nanowire heterostructures was attributed to the large lattice mismatch between CdSe and ZnS (12%), therefore making it much more complicated to produce than the quantum dot counterpart.[9]
As a result, there is a need for a coated nanowire material, specifically CdSe/ZnS nanowires, and method of making thereof that may be manufactured on a large scale, that provides reproducible products among various batches, and that generates nanowires that exhibit advantageous semiconducting properties. The method should be robust, and easily adaptable to other compositions, such as CdSe/CdS core/shell nanowires.
Additionally, there is a need for the ability to tune the morphology of the core/shell material for different applications. For example, a smooth shell is advantageous for applications that require higher luminescence yield. Conversely, a rougher higher surface area coating is advantageous for catalytic and sensing applications.